Multiple Input Multiple Output (MIMO) has recently attracted much attention as a broadband wireless mobile communication technology. A MIMO system increases data communication efficiency using a plurality of antennas. The MIMO system can be implemented in a MIMO scheme such as spatial multiplexing or spatial diversity depending on whether the same data is transmitted or not.
In spatial multiplexing, different data is transmitted simultaneously through a plurality of transmission antennas, thereby enabling high-speed data transmission without increasing a system bandwidth. Spatial diversity is a scheme that achieves transmit diversity by transmitting the same data through a plurality of transmission antennas. Space time channel coding is an example of spatial diversity.
In addition, MIMO schemes may be categorized into open-loop MIMO schemes and closed-loop MIMO schemes depending on whether a receiver feeds back channel information to a transmitter. The open-loop MIMO schemes include BLAST that can increase the amount of information as much as the number of transmission antennas by transmitting information in parallel from a transmitter and detecting a signal by repeating ZF (Zero Forcing) or MMSE (Minimum Mean Square Error) detection at a receiver and STTC (Space-Time Trellis Code) that can achieve transmit diversity and a coding gain by using a new spatial region. TxAA (Transmit Antenna Array) is a closed-loop MIMO scheme.
In a radio channel environment, a channel state changes irregularly in the time and frequency domains, which is called fading. Thus, a receiver corrects a received signal using channel information to recover data transmitted by a transmitter and detect a correct signal.
In a wireless communication system, a transmitter transmits to a receiver a signal known to both the transmitter and the receiver so that channel information is detected based on distortion that the signal has experienced during transmission on a channel. This signal is called a reference signal (or a pilot signal) and channel information detection is called channel estimation. The reference signal is transmitted with high power, without carrying actual data. When data is transmitted and received through a plurality of antennas, the channel state between each transmission antenna and each reception antenna should be determined and thus a reference signal exists for each transmission antenna.
A cooperative MIMO system was proposed to reduce inter-cell interference in a multi-cell environment. In the cooperative MIMO system, multi-cell Base Stations (BSs) can jointly support data for a UE. The BSs may also support one or more UEs, MS1, MS2, . . . , MSK simultaneously in the same frequency resources to increase system performance. Further, the BSs may perform Space Division Multiple Access (SDMA) based on channel state information between the BSs and a UE.
A serving BS and one or more cooperative BSs are connected to a scheduler through a backbone network in the cooperative MIMO system. The scheduler may operate based on feedback channel information about the channel states between the UEs, MS1, MS2, . . . , MSK and the cooperative BSs, measured at the BSs, BS1, BS2, . . . , BSM, received through the backbone network. For example, the scheduler schedules information required for a cooperative MIMO operation for the serving BS and the one or more cooperative BSs. That is, the scheduler directly issues commands regarding a cooperative MIMO operation to each BS.
CoMP was proposed to reduce inter-cell interference and improve the performance of UEs at a cell edge in the multi-cell environment. That is, a CoMP system can improve the communication performance of cell-edge UEs in the multi-cell environment. For this purpose, there is a need for accurate channel estimation based oh reference signals received from multi-cell BSs.
However, as more cells perform a CoMP operation, conventional CoMP reference signals have the shortcoming that a PN (Pseudo Noise) code is shortened within one resource block and lack of dispreading samples during channel estimation degrades channel estimation performance. Therefore, a new CoMP reference signal pattern is required to guarantee accurate estimation of channels from neighbor cells for a UE performing a CoMP operation.
Moreover, no CoMP reference signals have been defined so far for LTE-A (Long Term Revolution-Advanced).